In recent years, demand for miniaturization and reduction in cost has increased for mobile communication devices such as cellular phones. In order to meet this demand, direct conversion system receiving apparatuses of which the number of parts can be greatly reduced have come into wide use.
There is a problem, however, with the DC offset voltage of such direct conversion system receiving apparatuses. The DC offset voltage is a primary factor in damaging the dynamic range of amplifiers, A/D converters and the like which are used for signal processing of received signals and deteriorating the bit error rate, which is a scale for indicating the quality of reception. Direct conversion system receiving apparatuses and receiving circuits having a means for suppressing this DC offset voltage have already been proposed.
In the following, a direct conversion receiving circuit according to the prior art is described.
FIG. 2 shows an example of a direct conversion system receiving circuit according to the prior art. In FIG. 2, symbol 101 indicates a receiving circuit. Symbol 102 indicates an output terminal of receiving circuit 101. Symbol 104 indicates an input terminal of a receiving circuit 101. Symbol 105 indicates a mixer. Symbol 106 indicates an input terminal for a local oscillation signal. Symbol 107 indicates a variable gain amplifier. Symbol 108 indicates a low pass filter. Symbol 109 indicates a capacitor. Symbol 110 indicates a resistor. Symbol 111 indicates a high pass filter formed of capacitor 109 and resistor 110. Symbol 112 indicates a gain control apparatus. Symbol 114 indicates a reference voltage supply in high pass filter 111.
In addition, FIG. 5 shows the gain setting in a variable gain amplifier according to the prior art and chronological change in the output DC voltage of respective portions of the receiving circuit.
In FIG. 5, waveform a represents the chronological change in the gain setting of variable gain amplifier 107. In this gain setting, the set gain changes from gain G1 to gain G2 in step form at time t1.
Waveform b represents the chronological change in the output DC voltage of mixer 105. The output DC voltage of this mixer 105 has a value that is higher than the original operating point voltage (shown as broken line) because of DC offset voltage VOFS which exists in the output of mixer 105 and in the input of variable gain amplifier 107.
Waveform c represents the chronological change in the output DC voltage of variable gain amplifier 107. The output DC voltage of this variable gain amplifier 107 changes in step form in response to the change in the gain setting due to the existence of DC offset voltage VOFS in the output DC voltage of mixer 105. Here, the broken line indicates the output DC voltage of variable gain amplifier 107 in the case where DC offset voltage VOFS does not exist.
Waveform d represents the chronological change in the output DC voltage of low pass filter 108. The output DC voltage of this low pass filter 108 has a transient response waveform that responds to the change in step form in the output DC voltage of variable gain amplifier 107. Here, the broken line indicates the output DC voltage of low pass filter 108 when DC offset voltage VOFS does not exist.
Waveform e represents the chronological change in the output DC voltage of high pass filter 111. The transient response in the output DC voltage of low pass filter 108 remains in the output DC voltage of this high pass filter 111.
The operation of the direct conversion system receiving circuit formed as described above is described in the following.
In the direct conversion system receiving circuit, frequency conversion is carried out using a local oscillation signal that is inputted into input terminal for local oscillation signals 106, so that a high frequency signal that is inputted from signal input terminal 104 into mixer 105 is converted to a base band signal and then outputted. In the direct conversion system, however, the carrier frequency of this high frequency signal and the oscillation frequency of the local oscillation signal have the same frequency. Therefore, in the case where the local oscillation signal leaks to signal input terminal 104, for example, this leaked signal has the same frequency as the local oscillation signal, and therefore, is outputted as a DC offset voltage as a result of frequency conversion in mixer 105. In addition, a DC offset voltage is generated due to relative inconsistency in the circuit elements which form variable gain amplifier 107, low pass filter 108 and the like.
It is possible to remove a DC offset voltage that has been generated in this manner using high pass filter 111 which is formed of capacitor 109 and resistor 110. In addition, it is possible to sufficiently pass the base band signal by appropriately setting the cut off frequency of this high pass filter 111. Thus, it is possible to gain a demodulated signal without deteriorating the quality of the received signal (see for example Patent Document 1).    Patent Document 1: Japanese Unexamined Patent Publication H11 (1999)-225179